This invention relates generally to improvements to optical fiber, and more particularly to the design of a dispersion-compensating (DC) module.
Various mechanisms limit the bandwidth of an optical fiber. In a multimode optical fiber, for example, there is modal dispersion in which pulses of light that enter one end of the fiber are spread as they emerge from the other end of the fiber. This is because multimode fiber supports hundreds of different modes (or paths, if photons are viewed as particles rather than waves) as light propagates along the length of the fiber. Unfortunately, certain modes of consecutive pulses arrive at the distant end of the fiber at about the same time and interfere with each other. To avoid such intersymbol interference, the individual pulses of light in a multimode system are transmitted at a slower rate.
However, modal dispersion can be avoided with a fiber that is designed to support only the fundamental mode of a particular wavelength. Such a fiber is referred to as a singlemode fiber and has a much higher bandwidth than multimode fiber. But even here, a pulse of light that is introduced into one end of a singlemode fiber is somewhat spread in time as it emerges at the distant end. This is because the act of impressing information onto a single-wavelength lightwave (i.e., modulation) creates a spectrum of wavelengths that propagate along the fiber at different speeds. Accordingly, the different wavelength components (colors) that comprise the modulated lightwave arrive at the distant end of the fiber at different times, and the resulting pulse of light is xe2x80x9csmearedxe2x80x9d in time. Not surprisingly, this is referred to as chromatic dispersion.
A class of optical fibers has been developed that are known as dispersion-compensating (DC) fibers that, ideally, have dispersion characteristics that are opposite to the dispersion characteristics of the transmitting fiber. Reference is briefly made to FIG. 9, wherein curve 91 represents the chromatic dispersion of a known transmission fiber in the 1550 nanometers (nm) wavelength region. At this wavelength, the curve 91 shows that each kilometer (km) of fiber adds +17 ps (where, 1 ps=10xe2x88x9212 second) of dispersion of per nanometer of source spectral width. The positive (+) polarity merely indicates that wavelengths longer than 1550 nm travel more slowly than wavelengths that are shorter. For practical purposes, the polarity is meaningless. However, in order to compensate for this dispersion, a DC fiber is concatenated with the transmission fiber to add an equal but opposite amount of dispersion. Curve 92 represents the chromatic dispersion of a known DC fiber. At 1550 nm, this DC fiber provides xe2x88x9217 ps/nmxc2x7km of dispersion. Thus, if equal lengths of transmission fiber and DC fiber are concatenated, then the overall dispersion at 1550 nm will be zero. Unfortunately, wavelengths above and below 1550 nm will still experience a net amount of dispersion; and so a more desirable dispersion characteristic for a DC fiber is needed such as the one shown by curve 93, which not only has an opposite dispersion polarity with respect to the transmission fiber, but also has an opposite dispersion slope.
Indeed, for wavelength-division multiplexing (WDM) systems operating at 40 Gb/sec and above, it is necessary to compensate dispersion slope as well as dispersion polarity. Such compensation is achieved when the relative dispersion slope (RDS) of the DC fiber is equal to the RDS of the of the transmission fiber. RDS is defined as the dispersion slope divided by the dispersion (i.e., RDS=S/D). A recently developed, reduced-slope transmission fiber is disclosed in U.S. Pat. No. 5,878,182 which has a slope of only 0.045 ps/(nm2xc2x7km) and a dispersion of +4.5 ps/(nmxc2x7km) to yield an RDS of 0.01 nmxe2x88x921 at 1550 nm. Such a fiber is commercially available from OFS Fitel as its TrueWave(copyright) RS optical fiber. Nevertheless, even this fiber requires compensation, but there does not appear to be any commercially available DC fiber with an RDS greater than 0.0035 nmxe2x88x921.
In optical transmission systems, dispersion compensation is achieved by splicing a length of DC fiber to the transmission fiber as generally illustrated in FIG. 7 and, for convenience, the DC fiber is stored in a module. It is important that the insertion loss of the dispersion-compensating module be as low as possible for a number of reasons including: simpler amplifier design; lower signal-to-noise ratio of the transmission system; and reduced nonlinear effects because input power to the dispersion compensating module can be lower. The insertion loss of a dispersion-compensating module has two major contributors: (i) splice loss at the junction between the transmission fiber and the DC fiber; and (ii) the loss of the DC fiber itself. The insertion loss of the module can be lowered by using a DC fiber having a high figure of merit, which is defined as the ratio of the magnitude of the dispersion to the attenuation of the DC fiber. Unfortunately, state-of-the-art DC fibers have figures of merit that are less than 200 ps/(nmxc2x7dB).
U.S. Pat. No. 5,361,319 (Antos) discloses a DC fiber and system in which dispersion is compensated by inserting modules at appropriate intervals. Each module contains DC fiber of an appropriate length to produce a dispersion of approximate equal magnitude (but opposite polarity) to that of the transmission fiber in the route. Unfortunately, the DC fibers shown in Antos have relatively small negative chromatic dispersion (absolute valuexe2x89xa6100 ps/nmxc2x7km; exemplarily xe2x88x9265 ps/nmxc2x7km), necessitating the use of long lengths of DC fiber (e.g., 39 km of DC fiber to compensate the dispersion of 150 km of transmission fiber). Furthermore, the Antos technique apparently is practical only for dispersion compensation, with dispersion slope compensation being considered xe2x80x9cnot easily achieved in practicexe2x80x9d by the author.
U.S. Pat. No. 5,448,674 (Vengsarkar) discloses a DC fiber having relatively high dispersion (absolute value typically greater than 150 ps/nmxc2x7km) and a negative dispersion slope, both of which represent substantial improvements over Antos. In order to achieve such results, Vengsarkar""s DC fiber supports at least one higher order mode in addition to the fundamental LP mode (LP01). The drawback is the complication of adding mode converters, which potentially adds to the loss. Another difficulty is that the fibers are multi moded, meaning that modal noise due to interference between the modes degrades the signal-to-noise ratio.
Accordingly, what is needed is a DC fiber having high dispersion, a negative dispersion slope, and a high RDS. Satisfying these multiple goals in a singlemode optical fiber is a long sought-after goal of DC fiber designers.
A dispersion-compensating (DC) module comprises a first length of DC optical fiber in tandem with a second length of a standard singlemode optical fiber. The DC fiber has a relative dispersion slope (RDS) that is greater than 0.012 nmxe2x88x921.
In a preferred embodiment, the figure of merit (FOM) of the DC fiber is greater than 200 ps/(nmxc2x7dB) and the tandem combination of DC fiber and standard singlemode optical fiber has an RDS that is greater than 0.018 nmxe2x88x921. Preferably, the first length of DC fiber comprises at least 25% of the total of the first and second lengths of fiber on the module
Illustratively, the DC fiber is fabricated from silica glass and has a refractive index profile that includes a core region that is surrounded by a cladding region having a nominal refractive index n4. The core region includes a central core having a nominal refractive index n1, a xe2x80x9ctrenchxe2x80x9d surrounding the central core having a nominal refractive index n2, and a xe2x80x9cridgexe2x80x9d surrounding the trench having a nominal refractive index n3. A range of refractive index profiles has been found that provides desirable DC fibers. The range is conveniently expressed in terms of index differences:
0.015 less than n1xe2x88x92n4 less than 0.035;
xe2x88x920.012 less than n2xe2x88x92n4 less than xe2x88x920.006; and
0.002 less than n3xe2x88x92n4 less than 0.015;
This range of refractive index differences together with the below range of radial dimensions have been found to provide exceptionally high values of RDS (i.e., greater than 0.012 nmxe2x88x921):
In one illustrative embodiment, the DC module is used to compensate standard singlemode transmission fiber in an optical transmission system; and in another embodiment, DC fiber according to the invention is used to compensate non-zero dispersion shifted transmission fiber.